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C++ language tutorial

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C++ language tutorial Powered By Docstoc
					Structure of a program
Probably the best way to start learning a programming
language is by writing a program. Therefore, is our first
program:

// my first program in C++              Hello World!

#include <iostream>
using namespace std;

int main ()
{
  cout << "Hello World!";
  return 0;
}

The first panel shows the source code for our first
program. The second one shows the result of the program
once compiled and executed. The way to edit and compile a
program depends on the compiler you are using. Depending on
whether it has a Development Interface or not and on its
version. Consult the compilers section and the manual or
help included with your compiler if you have doubts on how
to compile a C++ console program.

The previous program is the typical program that programmer
apprentices write for the first time, and its result is the
printing on screen of the “Hello World!” sentence. It is
one of the simplest programs that can be written in C++,
but it already contains the fundamental components that
every C++ program has. We are going to look line by line at
the code we have just written:

//my first program in C++
     This is a comment line. All lines beginning with two
     slash signs (//) are considered comments and don’t
     have any effect on the behavior of the program. The
     programmer can use them to include short explanations
     or observations within the source code itself. In this
     case, the line is a brief description of what our
     program is.
#include<iostream>
     Lines beginning with a hash sign (#) are directives
     for the preprocessor. They are not regular code lines
     with expressions but indications for the compiler's
     preprocessor. In this case the directive #include
     <iostream> tells the preprocessor to include the
     iostream standard file. This specific file (iostream)
     includes the declarations of the basic standard input-
     output library in C++, and it is included because its
     functionality is going to be used later in the
     program.

using namespace std;
     All the elements of the standard C++ library are
     declared within what is called a namespace, the
     namespace with the name std. So in order to access its
     functionality we declare with this expression that we
     will be using these entities. This line is very
     frequent in C++ programs that use the standard
     library, and in fact it will be included in most of
     the source codes included in these tutorials.
int main ()
     This line corresponds to the beginning of the
     definition of the main function. The main function is
     the point by where all C++ programs start their
     execution, independently of its location within the
     source code. It does not matter whether there are
     other functions with other names defined before or
     after it - the instructions contained within this
     function's definition will always be the first ones to
     be executed in any C++ program. For that same reason,
     it is essential that all C++ programs have a main
     function.

    The word main is followed in the code by a pair of
    parentheses (()). That is because it is a function
    declaration: In C++, what differentiates a function
    declaration from other types of expressions are these
    parentheses that follow its name. Optionally, these
    parentheses may enclose a list of parameters within
    them.

    Right after these parentheses we can find the body of
    the main function enclosed in braces ({}). What is
    contained within these braces is what the function
    does when it is executed.
cout << "Hello World!";
     This line is a C++ statement. A   statement is a simple
     or compound expression that can   actually produce some
     effect. In fact, this statement   performs the only
     action that generates a visible   effect in our first
     program.

    cout represents the standard output stream in C++, and
    the meaning of the entire statement is to insert a
    sequence of characters (in this case the Hello World
    sequence of characters) into the standard output
    stream (which usually is the screen).

    cout is declared in the iostream standard file within
    the std namespace, so that's why we needed to include
    that specific file and to declare that we were going
    to use this specific namespace earlier in our code.

    Notice that the statement ends with a semicolon
    character (;). This character is used to mark the end
    of the statement and in fact it must be included at
    the end of all expression statements in all C++
    programs (one of the most common syntax errors is
    indeed to forget to include some semicolon after a
    statement).
return 0;
     The return statement causes the main function to
     finish. return may be followed by a return code (in
     our example is followed by the return code 0). A
     return code of 0 for the main function is generally
     interpreted as the program worked as expected without
     any errors during its execution. This is the most
     usual way to end a C++ console program.
You may have noticed that not all the lines of this program
perform actions when the code is executed. There were lines
containing only comments (those beginning by //). There
were lines with directives for the compiler's preprocessor
(those beginning by #). Then there were lines that began
the declaration of a function (in this case, the main
function) and, finally lines with statements (like the
insertion into cout), which were all included within the
block delimited by the braces ({}) of the main function.
The program has been structured in different lines in order
to be more readable, but in C++, we do not have strict
rules on how to separate instructions in different lines.
For example, instead of

     int main ()
     {
       cout << " Hello World!";
       return 0;
     }


We could have written:

     int main () { cout << "Hello World!"; return 0; }


All in just one line and this would have had exactly the
same meaning as the previous code.

In C++, the separation between statements is specified with
an ending semicolon (;) at the end of each one, so the
separation in different code lines does not matter at all
for this purpose. We can write many statements per line or
write a single statement that takes many code lines. The
division of code in different lines serves only to make it
more legible and schematic for the humans that may read it.

Let us add an additional instruction to our first program:

// my second program in C++          Hello World! I'm a C++ program

#include <iostream>

using namespace std;

int main ()
{
  cout << "Hello World! ";
  cout << "I'm a C++ program";
  return 0;
}

In this case, we performed two insertions into cout in two
different statements. Once again, the separation in
different lines of code has been done just to give greater
readability to the program, since main could have been
perfectly valid defined this way:
   int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; }



We were also free to divide the code into more lines if we
considered it more convenient:

    int main ()
    {
      cout <<
        "Hello World!";
      cout
        << "I'm a C++ program";
      return 0;
    }


And the result would again have been exactly the same as in
the previous examples.

Preprocessor directives (those that begin by #) are out of
this general rule since they are not statements. They are
lines read and processed by the preprocessor and do not
produce any code by themselves. Preprocessor directives
must be specified in their own line and do not have to end
with a semicolon (;).

Comments

Comments are parts of the source code disregarded by the
compiler. They simply do nothing. Their purpose is only to
allow the programmer to insert notes or descriptions
embedded within the source code.

C++ supports two ways to insert comments:

    // line comment
    /* block comment */


The first of them, known as line comment, discards
everything from where the pair of slash signs (//) is found
up to the end of that same line. The second one, known as
block comment, discards everything between the /*
characters and the first appearance of the */ characters,
with the possibility of including more than one line.
We are going to add comments to our second program:
/* my second program in C++                                  Hello World! I'm a C++ program
   with more comments */

#include <iostream>
using namespace std;

int main ()
{
  cout << "Hello World! ";     // prints Hello World!
  cout << "I'm a C++ program"; // prints I'm a C++ program
  return 0;
}

     If you include comments within the source code of your
     programs without using the comment characters combinations
     //, /* or */, the compiler will take them as if they were
     C++ expressions, most likely causing one or several error
     messages when you compile it.

				
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